1. Field of the Invention
The present invention relates to a clock spring which is used in a steering system of a vehicle to establish an electrical connection between a stationary member and a movable member by means of a flexible cable.
2. Description of Related Art
A clock spring comprises a stationary member, a movable member rotatably supported relative to the stationary member and a flexible cable connecting these members together, and is used as an electrical connection device for, e.g., a steering system of a vehicle
A clock spring of this type includes, as is already known, a flat cable received in a cable receptacle between a stationary member and a movable member The flat cable comprises a plurality of electrically conductive wires laminated with a pair of narrow strips of film, and is loosely received in the receptacle as the cable is spirally wound. The outer end of the flat cable is fixed to an outer annular wall formed on either the stationary or movable member, with the inner end being fixed to an inner annular wall formed on the other member.
The clock spring is such tat, when the movable member is rotated relative to the stationary member, a part of the flat cable, loosely received in the cable receptacle, is either wound onto the inner annular wall or unwound therefrom, depending on the direction in which the movable member is rotated. Thus, the part of the cable between its inner and outer ends can be displaced toward either the inner annular wall or the outer annular wall. The flat spring is substantially free from tensile force if the displacement takes place within the range between the position where the greater part of the flat cable is completely wound on the inner annular wall and the position where it is completely unwound therefrom. In this way, the flat cable acts to maintain an electrical connection between the stationary and movable members which rotate relative to each other.
In the clock spring, the flat cable is wound or unwound by utilizing the difference in diameter between the outer annular wall and the inner annular wall. Therefore, if the amount of rotation of the movable member is the same, a greater difference in the diameter allows a greater reduction in the length of the flat cable. However, the diameter of the inner annular wall is determined, in a one-to-one relation, by the diameter of the rotary shaft, such as the steering shaft of the vehicle, to which the clock spring is mounted. On the other hand, the diameter of the outer annular wall cannot be made very large in view of the requirement that the entire apparatus should be as small as possible. For these reasons, the difference between the diameter of the outer annular wall and that of the inner annular wall cannot be very large. As a result, it has generally been required that a clock spring of the type being discussed have a considerably long flat cable. A long flat cable, such as that in the known clock spring, involves various manufacturing difficulties, which inevitably increase the total production cost of the clock spring.